Adding meta transactions for deposit (EVM)

[mrwillis]

Motivation

Meta transactions for deposits would be really helpful for this project given its popularity. They essentially allow people to use EVM compatible chains without having the native token, and this is especially useful for first time users of the chain as they will have no native balance on that chain. A classic example is the following:

1. User withdraws ERC20 assets from an exchange to their wallet on chain A
2. Wants to bridge to chain B but not having any native tokens to do so
3. Asks friends on chain A for a few cents of the native token in order to bridge, leading to frustration and bad UX.

I floated the idea a few months ago in the discord but we now have the resources to help out with it.

We at SX Network would be happy to tackle the implementation given some guidance regarding PR acceptance criteria.

Technical discussion

The obvious place where this would be the most useful would be the deposit function

https://github.com/ChainSafe/chainbridge-solidity/blob/master/contracts/Bridge.sol#L328

The main issue here is that the method is payable for the fee. With a meta txn, this won't work so the fee would probably have to be taken in-kind. Then there's also the question of how does that fee work, is it %, flat, etc.

Feels like the most obvious/simple thing to do would be a flat fee map from token address -> fee amount stored in the handler contracts.

https://github.com/ChainSafe/chainbridge-solidity/blob/master/contracts/handlers/ERC20Handler.sol#L14

Apart from that, it looks like the GSN Context is already used in some places

https://github.com/ChainSafe/chainbridge-solidity/blob/6869c8db364926dc8a8fec83cd29e14f8ff532d8/contracts/utils/AccessControl.sol

which will make it relatively easy to integrate.

Looking forward to hearing others' thoughts!

[P1sar]

Hey! Right now we are concentrated on our new bridge version https://github.com/ChainSafe/chainbridge-core.
And this one is no longer will be supported in terms of new features. So you should take a look on core is has pretty much changes, one of them is it made more modular so it is possible to bring you own module that will support Meta transactions.
Actually right now we are working on adding infura ITX support. But we are more concentrated on adding ITX for the Voting rather than Depositing.
Fee for deposits is not widely used right now and we are still working on it concept, so probably it could be leaved as it is.
If you want to add Meta support ofr deposits you need to check our https://github.com/ChainSafe/chainbridge-ui which is basically Deposit sender